adding NIX_CONTINUE_DOWNLOADS feature, see mkdir comment

svn path=/nixpkgs/trunk/; revision=16401

pkgs/build-support/fetchurl/builder.sh

@@ -23,8 +23,19 @@ tryDownload() {
     echo
     header "trying $url"
     success=
-    if $curl --fail "$url" --output "$out"; then
+    cache_file="/var/nix-downloads/$outputHash"
-        success=1
+    if { test -n "$NIX_CONTINUE_DOWNLOADS" && touch "$cache_file";}; then
+      chmod g+w $cache_file # ensure another nixbld user can continue!
+      # no locking is taking place. I think nix builder locking is enough
+      if $curl --fail "$url" -C - --output "$cache_file"; then
+          mv "$cache_file" "$out"
+          chmod g-w $out # ensure another nixbld user can continue!
+          success=1
+      fi
+    else
+      if $curl --fail "$url" --output "$out"; then
+          success=1
+      fi
     fi
     stopNest
 }

pkgs/build-support/fetchurl/default.nix

@@ -1,7 +1,7 @@
 # Argh, this thing is duplicated (more-or-less) in Nix (in corepkgs).
 # Need to find a way to combine them.
 
-{stdenv, curl}: # Note that `curl' may be `null', in case of the native stdenv.
+{stdenv, curl, getConfig ? (x: y : "")}: # Note that `curl' may be `null', in case of the native stdenv.
 
 let
 
@@ -86,6 +86,18 @@ stdenv.mkDerivation {
       if sha256 != "" then "sha256" else if sha1 != "" then "sha1" else "md5";
   outputHash = if outputHash != "" then outputHash else
       if sha256 != "" then sha256 else if sha1 != "" then sha1 else md5;
+
+
+ 
+  # set this to any value to make nix dowload into /var/nix-downloads/$hash
+  # so that it can continue a half finished download after a shudown,
+  # susupend to disk, shutdown etc
+  # for this to work you have to run
+  # d=/var/nix-downloads; mkdir $d; chgrp nixbld $d; chmod g+x $d;
+  # once
+  # defaulting to enabled because continuing takes place only if $d exists
+  # and has proper permissions
+  NIX_CONTINUE_DOWNLOADS = getConfig ["NIX_CONTINUE_DOWNLOADS"] "1";
   
   impureEnvVars = [
     # We borrow these environment variables from the caller to allow

pkgs/top-level/all-packages.nix

@@ -282,7 +282,7 @@ let
   # from being built.
   fetchurl = useFromStdenv "fetchurl"
     (import ../build-support/fetchurl {
-      inherit curl stdenv;
+      inherit curl stdenv getConfig;
     });
 
   # fetchurlBoot is used for curl and its dependencies in order to